1. Technical Field
The present invention relates to a power amplifier.
2. Description of the Related Art
A Wideband Code Division Multiple Access (WCDMA) power amplifier requires a linear power of 28.5 dBm and an output of a maximum of 32 dBm.
Furthermore, the IMD3 linear characteristic must be equal to or higher than 25 dBc.
However, when a WCDMA power amplifier is implemented using a CMOS, a gate oxide film breaks down due to load mismatch, or a punch through between a drain and a source occurs, thereby breaking down a circuit.
Therefore, it is required to design a power amplifier considering the margin adjustable by a CMOS device as well as the maximum output power.
Up to the present, the breakdown of the circuit has been avoided by using a cascode power amplifier which uses a transistor having a short gate length (a thin oxide) as a common source and uses a transistor having a long gate length (a thick oxide) as a common gate, as shown in FIG. 1.
FIG. 1 is a diagram showing a conventional cascode power amplifier.
The conventional power amplifier includes a common source transistor M1 and a common gate transistor M2, as shown in FIG. 1, and is configured in a cascode structure in which the drain terminal of the common source transistor M1 and the source terminal of the common gate transistor M2 are connected to each other.
In the power amplifier having a cascode structure, the common gate transistor M2 prevents the common source transistor M1 from being degraded through breakdown by voltage applied to a drain terminal, which is an output terminal Vout, thereby enabling stable amplification.
For this purpose, the conventional power amplifier is configured such that the drain terminal of the common source transistor M1 is connected to the source terminal of the common gate transistor M2, the source terminal of the common source transistor M1 is connected to a ground GND, and the gate terminal of the common source transistor M1 is connected to an input terminal Vin.
Meanwhile, a first capacitor C1 is disposed between the gate terminal of the common source transistor M1 and the input terminal Vin, and removes noise from an input signal sent from the input terminal Vin.
Furthermore, a first bias power source Vg1 for driving the common source transistor M1 and a first resistor Rg1 are connected between the gate terminal of the common source transistor M1 and the ground GND.
The drain terminal of the common gate transistor M2 is connected to a driving voltage VDD, the source terminal of the common gate transistor M2 is connected to the drain terminal of the common source transistor M1, and the gate terminal of the common gate transistor M2 is connected to a second bias power source Vg2 via a second resistor Rg2.
Meanwhile, the drain terminal of the common gate transistor M2 is connected to the output terminal Vout of the power amplifier, a third capacitor Cm for removing noise from an output amplified signal is disposed between the drain terminal of the common gate transistor M2 and the output terminal Vout, and a matching circuit or a power connection terminal is connected to the output terminal Vout.
Furthermore, a second capacitor C2 for removing noise from a signal is connected between the gate terminal of the common gate transistor M2 and the source terminal of the common source transistor M1.
In the conventional cascode power amplifier, the common gate transistor M2 is generally formed of an NMOSFET having a long gate length, that is, a thick oxide film, so as to improve the breakdown characteristic of the common source transistor M1, and the common source transistor M1 is formed of an NMOSFET having a short gate length, that is, a thin oxide film, so as to increase an amplification factor for an input signal.
Accordingly, the conventional cascode power amplifier has the amplification gain and harmonic characteristic shown in FIGS. 2 and 3.
Referring to FIGS. 2 and 3, the conventional cascode power amplifier has an amplification gain of 10.003 dBm, a power efficiency of 45.739%, and a harmonic characteristic of −32.437 dBm at an output power of 25.503 dBm.
The conventional cascode power amplifier is configured such that the voltage difference between the gate and drain of the common source transistor M1 is greater than the voltage difference between the gate and drain of the common gate transistor M2. This difference increases in proportion to output power, and must be maintained at a specific voltage so as to prevent the circuit from breaking down.
Accordingly, in order to improve a breakdown characteristic, a common source transistor M1 having a thick oxide film must be used. However, when the common source transistor M1 having a thick oxide film is used, the breakdown characteristic can be improved, but the gain characteristic is insufficient, so that desired output power cannot be achieved.